Go to router IP 192.168.0.1 and log in with your credentials.
Then find out the IP of that client you want to limit, so go to DHCP>> DHCP Client List. There you will see the list of the currently active user on your Wifi and Assigned IP addresses to those devices.
Select an IP which you identified to limit bandwidth.
Now go to Bandwidth Control>>Rule List.
Now put the values in boxes, such as IP address of the client in IP range and Bandwidth limit in Egress Bandwidth & Ingress Bandwidth both same value in Min and Max option. Leave other fields blank.
Click Save.
Ps. Palitan mo nalang yung ip ng default gateway mo depende sa router na gagamitin mo